1. Field of the Invention
This invention relates to a projection exposure apparatus, and in particular to improvements in the alignment system in the projection exposure apparatus.
2. Related Background Art
Of the alignment systems for the alignment of a mask (or a reticle) and a wafer effected in a projection exposure apparatus, in the system using alignment light of a wavelength differing from the wavelength of exposure light, the mask and the wafer do not always assume conjugate positions for the wavelength of the alignment light even if the mask and the wafer are provided at conjugate positions for the wavelength of the exposure light. For this reason, heretofore, there has been practised the countermeasure of placing the mask and the wafer in a conjugate relation even for the alignment light by the insertion of a correction lens or the insertion of an auxiliary mirror for changing the length of the optical path.
However, in the above-described countermeasure using the insertion of the correction lens or the change of the length of the optical path, the inserted position of the correction lens or the set position of the optical path is liable to shift, and there has also been an inconvenience that the alignment position, i.e., the position of the alignment mark of the mask, is fixed relative to the exposure optic axis and the position of the mark cannot be changed in conformity with the size of the pattern.
Also, when light of the same wavelength as the exposure light is used as the alignment light, there is an inconvenience that a photosensitive medium such as the photoresist on the wafer which is a semiconductive wafer is sensitized during alignment and the mark used for the alignment cannot be reused. Particularly, in a case where photoresist having mixed therewith a coloring matter which absorbs and does not reflect the exposure light is used as the photoresist, there occurs an inconvenience that alignment becomes difficult.